Subsequently published DE 102 47 353 discloses a process for reducing the flow dependence of measuring instruments for determination of impurities as indication of the quality of a fluid, especially solid impurities such as particles in fluids. A particle count sensor operates in particular on the basis of the light blocking principle, and is mounted in a measuring cell of the measuring device. The measuring device has a specifiable input cross-section for the flow of fluid. The sensor generates a light beam cross-sectional area over which the flow of fluid is conducted for detection of an impurity. Particle count sensors operating on the light blocking principle determine the relative proportion of the light beam cross-sectional area (perpendicular to the optical axis) covered by projection of a pollutant particle in this plane.
DE 198 60 169 A1 discloses a process for qualitative determination of small amounts of water in multicomponent systems in the liquid state of aggregation, oil in particular. The process is characterized by repetition of the following process steps several times:                incomplete extraction of moisture from the multicomponent system by a carrier gas;        quantitative determination of the amount of moisture extracted by measurement of the relative humidity in the carrier gas, the carrier gas volume, and the temperature; and        conversion to the amount of moisture of the multicomponent system after determination of the mass of the multicomponent system and the saturation vapor density in the carrier gas.        
The disclosed process and device permit measuring the absolute saturation concentration of moisture in fluids such as hydraulic oil. The parameter determination in question in turn permits formulation of a statement regarding the quality of the oil.
DE 101 52 777 A1 discloses a device for determination of the quality of a medium, a lubricant and/or cutting oil in particular, having several sensors generating an electric output signal as a function of the respective sensor-specific input quantity. One sensor is a temperature sensor generating an output signal which is a function in essence exclusively of the temperature of the medium and is essentially independent of the quality of the medium. At least one other sensor generates an output signal as a function both of the quality of the medium (fluid) and of the temperature of the medium. These sensors are mounted on a common substrate immersible in the fluid, so that the measuring device is mounted in a very small structural space.
The disclosed measuring devices and processes indicated in the foregoing make available a very good set of instruments for determination of quality parameters for fluids, including media in the form of a gas and/or paste. Those measuring devices may also be supplemented by chemical analysis processes, for example, in order to arrive at determinations regarding free radicals in a hydraulic oil, temperature, viscosity, pH value, electric conductivity, etc. Such devices entail a prolonged measurement or determination period depending on the measuring process employed, along with the associated measurement device and depending on the quality parameters of the fluid to be determined. Out of consideration of reasons of process accuracy, length of the measurement period, and validity of the test result, it has been found to be advisable to use such quality measurement processes directly on test stands where the operation of fluid devices such as hydraulic assemblies, valves, filter housings, flexible pressure tubing, etc., may be tested. Use of the operating fluid is necessary, on location, to verify the quality of the fluid used by the measuring device, if possible simultaneously with testing of the assembly. In this way, determination is made of the suitability of the fluid device for subsequent operation, in turn, to obtain a broader indication of the quality of the preceding production steps with respect to the fluid device (assembly) which has been or is to be tested.